dtoc: Support packing the device tree

After any node/property deletion the device tree can be packed to remove
spare space. Add a way to perform this operation.

Note that for fdt_fallback, fdtput automatically packs the device tree after
deletion, so no action is required here.

Signed-off-by: Simon Glass <sjg@chromium.org>

lib/libfdt/libfdt.swig

@@ -107,3 +107,4 @@ int fdt_next_subnode(const void *fdt, int offset);
 int fdt_delprop(void *fdt, int nodeoffset, const char *name);
 
 const char *fdt_strerror(int errval);
+int fdt_pack(void *fdt);